In the search for high performance materials, considerable interest has been focused upon carbon fibers. The terms "carbon" fibers or "carbonaceous" fibers are used herein in the generic sense and include graphite fibers as well as amorphous carbon fibers.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength and high modulus. During such service, the carbon fibers commonly are positioned within the continuous phase of a resinous matrix (e.g. a solid cured epoxy resin). Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels, ablative materials for heat shields on re-entry vehicles, strong lightweight sports equipment, etc.
As is well known in the art, numerous processes have heretofore been proposed for the thermal conversion of organic polymeric fibrous materials (e.g. an acrylic multifilamentary tow) to a carbonaceous form while retaining the original fibrous configuration substantially intact. During commonly practiced carbon fiber formation techniques, a multifilamentary tow of substantially parallel or columnized carbon fibers is formed with the individual "rod-like" fibers lying in a closely disposed side-by-side relationship. See for instance, the following commonly assigned U.S. Pat. Nos. 3,539,295; 3,656,904; 3,723,157; 3,723,605; 3,775,520; 3,818,082; 3,844,822; 3,900,556; 3,914,393; 3,925,524; 3,954,950; and 4,020,273.
In addition to carbon fibers, there has been interest in the use of ceramic materials, including ceramic fibers for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness.
Among the ceramic materials which have been suggested are those made from organosilicon polymers. Thus, polymers based on silicon, carbon and/or nitrogen and/or oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes and the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
Other metallic polymers have been suggested as ceramic precursers. Thus, U.S. Pat. No. 4,581,468 forms boron nitride by pyrolyzing B-triamino-N-tris (trialkylsilyl)borazines. U.S. Pat. No. 4,097,294 suggests that a boron carbide ceramic is obtainable from a carborane carbon polymer.
The formation of aluminum nitride fibers is disclosed in commonly assigned, U.S. Pat. No. 4,687,657. Aluminum nitride ceramics are formed by thermal conversion of poly-N-alkyliminoalanes. Ceramics comprising silicon carbide and aluminum nitride solid solutions are also disclosed. These ceramic alloys are formed by thermal conversion of a mixture of an organosilicon preceramic polymer and the above-mentioned aluminum-containing polymer. Moreover, many recent patents describe specific silicon-containing preceramic polymers which are formed into silicon carbide and/or nitride upon thermal treatment.
Alternatively, ceramic fibers such as metal carbide fibers have been formed by incorporating inorganic metallic compounds into a carbon fiber product, the precarbonaceous polymer forming solution, the polymer spinning solution or the polymer fiber subsequent to spinning, and converting the metallic compounds in situ to metal carbides upon thermal conversion. In these methods, the precarbonaceous polymer acts as the source of carbon.
Important ceramics formed by such method are boron carbide and boron carbide-containing carbon fibers. The addition of boron carbide to carbon fiber is known to increase fiber strength and, more particularly, to substantially increase the thermo-oxidative stability of carbon fibers such that the boron carbide-containing carbon fibers can withstand higher temperature environments than carbon fibers. Methods of incorporating boron into carbon fibers to form boron carbide fibers have typically involved treating the carbon fibers with gaseous boron halides or impregnation with soluble borane salts or boric oxides including boric acid, metallic borates and organic borates, e.g. alkyl and aryl borates. Upon being treated with the boron compounds, the fibers are heated to initiate reaction of boron with the carbon fibers to yield boron carbide.
In commonly assigned, copending application U.S. Ser. No. 082,761, filed Aug. 7, 1987, now U.S. Pat. No. 4,832,895, May 23, 1989 boron-containing fibers are provided by forming a blend of a boron-containing polymer and a precarbonaceous polymer, shaping the blend into a fiber such as by spinning and pyrolyzing to form a boron ceramic fiber. Preferably, the boron-containing polymers are prepared by the condensation of boranes with Lewis bases. Such polymers are well known and prepared by condensing a borane such as diborane, pentaborane or decaborane with Lewis bases such as amines, amides, isocyanates, nitriles and phosphines. A particularly preferred borane-containing polymer is one formed by the condensation of decaborane and dimethylformamide (DMF). The borane-Lewis base condensation polymers are known and described, for example, in POLYMER LETTERS, Vol. 2, pp. 987-989 (1964); Chemical Society (London) Spec. Publ. No. 15 (1961), "Types of Polymer Combination among the Non-metallic Elements", Anton B. Burg, pp. 17-31; U.S. Pat. Nos. 2,925,440; 3,025,326; 3,035,949; 3,071,552; and British Patent No. 912,530. Other borane-containing polymers suggested include those disclosed in U.S. Pat. No. 3,441,389 wherein borane polymers are prepared by heating a compound of the formula (RAH.sub.3).sub.2 B.sub.10 H.sub.10 or (RAH.sub. 3).sub.2 B.sub.12 H.sub.12 at a temperature of 200.degree.-400.degree. C. for several hours. Moreover, borazines such as disclosed in U.S. Pat. No. 4,581,468 and carborane polymers such as suggested in U.S. Pat. No. 4,097,294 are also considered useful.
The use of organometallic polymers as precursors for ceramic materials is advantageous in the formation of ceramic fibers. It is considerably easier to spin the polymeric materials than inorganic precursors composed of inorganic metallic particles dispersed in a spinnable organic matrix. It would, therefore, be desirable to find new organometallic polymers and methods of making same which can be used as ceramic precursors. The present invention is concerned with preparing organoboron polymers which can serve as precursors for boron ceramics such as boron carbide and boron nitride and ultimately to the formation of fibers of these boron-containing ceramic materials.
One difficulty in preparing boron-containing ceramics from organic precursers is the inability to incorporate sufficient boron into the organic polymer and react with the carbon components to form boron carbide, B.sub.4 C. Methods of incorporating boron-containing salts or boron-containing inorganic powders and the like into precarbonaceous polymer solutions, solids, or the formed carbon articles have proved unsuccessful in providing sufficient amounts of boron to yield improved boron carbide-containing ceramic materials. The boron-containing polymers as described in the aforementioned commonly assigned, co-pending applications have yielded boron carbide ceramics containing greater than 40% boron. There is, however, a continuing need to find additional preceramic organoboron polymeric materials which yield ceramics containing increased levels of boron.
As described previously, decaborane-containing polymers such as those produced by the reaction of decaborane with a Lewis base are known. Additionally, organoborane polymers have been produced by polymerizing difunctional carboralated acetylene monomers such as by condensation.
Carborane which is a compound of carbon, hydrogen and boron has the empirical formula C.sub.2 H.sub.12 B.sub.10. While there is some difference of opinion as to the molecular structure of carborane, its stability is usually attributed to a basket-shaped molecular configuration in which the 10 boron atoms and 2 carbon atoms are arranged at the apices of an icosahedron. The following formula has been proposed wherein the circle indicates generalized, delocalized pi-bonding between the carbon and boron atoms. ##STR1## The carboralated monomers have been prepared by first reacting decaborane with an electron-donor compound, e.g., acetonitrile, to form a coordination compound, e.g. (CH.sub.3 CN).sub.2 B.sub.10 H.sub.12, bis-(acetonitrilo) decaborane. The coordination compound is then reacted with a compound having acetylenic unsaturation to form a carborane derivative. A method of forming polyester carboranes is disclosed in U.S. Pat. No. 3,351,616. Other patents disclosing carborane compounds include U.S. Pat. Nos. 3,217,031; 3,247,256; 3,254,117; 3,234,288; 3,359,304; and 3,505,409; all of which are herein incorporated by reference. The boron-containing compounds and polymers disclosed in the aforementioned patents are used primarily as high energy fuels such as for rocket propellants.
It is also known to prepare a biscarborane by reacting diacetylene with bis(acetonitrile) decaborane to carboralate one of the acetylenic bonds and additionally reacting with bis(acetonitrile) decaborane to carboralate the second acetylenic bond. As far as is known, the carboralated acetylenic including diacetylenic compounds and polymers as above-described have not previously been suggested as precursers for boron-containing ceramic materials.